Researchers from Chonnam National University in the Republic of Korea report that a nanometer-scale germanium oxide layer can address a long-standing bottleneck in tin monosulfide thin-film solar cells by improving the rear contact surface with the metal electrode. They focused on tin monosulfide, or SnS, a non-toxic and inexpensive absorbent that uses elements abundant in the Earth and avoids indium, gallium and tellurium, while theoretically providing favorable optical and electronic properties for harvesting sunlight. In practice, measured efficiencies have fallen short of theoretical predictions due to structural defects, parasitic reactions, and atomic diffusion at the interface where SnS meets the contact with the backmetal.
The team led by Professor Jaeyeong Heo and Dr Rahul Kumar Yadav designed a device architecture that inserts an ultra-thin germanium oxide or GeOx interlayer between the molybdenum back contact and the SnS absorber. Their study, published online in Small on September 19, 2025, describes how this interface engineering strategy targets deep defects and unwanted chemical phases that form during high-temperature processing of conventional SnS cells.
To fabricate the GeOx layer, the researchers used a vapor transport deposition process that first deposits a very thin germanium film and then relies on its natural oxidation to form a controlled oxide with a thickness of about 7 nanometers. This approach is compatible with scalable, industry-oriented thin-film processing. “Despite its nanoscale thickness, this interlayer addresses several long-standing challenges simultaneously,” explains Prof. Heo. “It suppresses damaging deep defects, blocks unwanted sodium diffusion and prevents the formation of resistive molybdenum disulfide phases during high-temperature manufacturing.”
By stabilizing the rear interface, the GeOx layer improves the microstructure of the SnS absorber, creating larger and more uniform grains that facilitate charge transport. The authors report improved charge collection and lower electrical losses, increasing the overall performance of the device. In quantitative terms, the energy conversion efficiency increased from 3.71% in standard devices without the interlayer to 4.81% when the optimized GeOx passivation was implemented, one of the highest reported efficiencies for vapor-deposited SnS-based solar cells.
The work also highlights broader implications of precise control over metal-semiconductor interfaces beyond photovoltaic devices. The researchers note that similar interface optimization can influence contact resistance and switching behavior in thin-film transistors, energy conversion performance in thermoelectric modules, charge transfer and detection sensitivity in sensors, mechanical reliability in flexible electronics, and the operation of photodetectors and memory devices. “Across all these applications, mastering the metal/semiconductor interface remains crucial for advancing next-generation devices,” says Prof. Heo. “We believe this work will open new avenues for research and contribute to the development of advanced solar cells and other key technologies.”
Research report:Optimized backside passivation of SnS thin-film solar cells using a controlled germanium oxide interlayer for improved photovoltaic performance
